This application is related to copending patent application Ser. No. 07/314,747, filed on Feb. 24, 1989, and assigned to the same assignee.
1. Field of the Invention
The present invention relates generally to floating structures and, more particularly, to oil-and-gas drilling and production platforms using onboard tensioners for tensioning production risers, which extend offshore wells to wellheads on the platforms.
2. Description of the Prior Art
A platform is effectively a spring mass system and as such has a resonant (natural) frequency F.sub.n or period T.sub.n =l/F.sub.n and is subject to resonant oscillatory motion in response to wave action in the seaway. Resonant motion occurs when the natural period of heave is substantially equal to the period of the wave which induces such heave in the platform.
The patent literature describes various structures and arrangements for dynamically and passively damping a floating platform, but these usually require design changes to the platform itself, and/or the use of special devices for achieving the desired damping.
For example, Bergman's U.S. Pat. No. 4,167,147 describes a floating structure having a variety of arrangements for producing velocity damping, i.e., anti-heave forces that are proportional to the heave velocity of the structure.
Bergman's damping system is intended to exert anti-heave forces as the vessel heaves up and also as the vessel heave down. These anti-heave forces are exerted on the structure in a direction opposite to its vertical motion; they are much smaller than the actual wave forces which produce the heave; and they provide a most effective decrease in heave amplitude, especially when the platform is about to approach resonance.
Bergman illustrates in FIG. 14 a passive, damping system which requires a tensioned flexible cable the lower end of which is anchored to a weight on the sea floor, and its upper end passes over a sheave and is fixedly secured to the platform's upper deck. Also on the upper deck is a hydraulic cylinder whose piston rod supports the sheave. The cylinder is filled with pressurized oil below the piston. A restrictive orifice is interposed in the pipe between an oil reservoir and the cylinder to restrict the oil flow between the cylinder and the reservoir.
A deep-floating production platform, which produces oil through wellheads suspended above the waterline, must make use of one production riser for each suspended wellhead. Each riser tensioner system comprises at least one hydraulic cylinder, and a pneumatic-hydraulic source for supplying pressurized fluid to the cylinder. The cylinder is extensibly coupled between a deck and a guide ring which is pivotably anchored to the upper end of the production riser.
This tensioner system is designed to maintain a predetermined minimum, nearly constant tension in the production riser despite relative vertical movement between the floating platform and the guide ring in response to oscillatory wave action on the platform.
It is an object of the present invention to prevent excessive platform resonant heave by modifying the already existing riser tensioner system so that it can generate and apply a downward-acting, anti-heave force to the platform, without interfering with the tensioner's ability to maintain the predetermined minimum tension sufficient to prevent buckling in the production riser, while continuous fluid production takes place from the well through the riser and its associated wellhead tree.
These downward-acting, anti-heave forces can be generated using hydraulic, mechanical, and/or electrical damping means, which maintain, within acceptable limits, the resonant heave response of the platform to wave energy exceeding the expected maximum wave period.